Bushbaby growth hormone is much more similar to nonprimate growth hormonesthan to rhesus monkey and human growth hormones

Unlike other mammals, Old World primates have five growth hormone-like gene s that are highly divergent at the amino acid level from the single growth hormone genes found in nonprimates. Additionally, there is a change in the interaction of growth hormone with its receptor in humans such that human g rowth hormone functions in nonprimates, whereas nonprimate growth hormone i s ineffective in humans. A Southern blotting analysis of the genome of a pr osimian, Galago senegalensis, revealed a single growth hormone locus. This single gene was PCR-amplified from genomic DNA and sequenced. It has a rate of nonsynonymous nucleotide substitution less than one fourth that of the human growth hormone gene, while the rates of synonymous substitution in th e two species are less different. Human and rhesus monkey growth hormones e xhibit variation at a number of amino acid residues that can affect recepto r binding. The galago growth hormone is conservative at each of these sites , indicating that this growth hormone is functionally like nonprimate growt h hormones. These observations indicate that the amplification and rapid di vergence of primate growth hormones occurred after the separation of the hi gher primate lineage from the galago lineage.